A cell will be over-charged if the charging voltage on the cell exceeds the upper limit of charging voltage when the cell is charged; a cell will over-discharge if the discharging voltage of the cell is lower than the lower limit of discharging voltage when the cell discharges. For example, for a Li-ion cell, if the Li-ion cell is over-charged, the pressure in the cell will increase, the cell will be deformed, electrolyte leakage may occur, and the cell may even explode or burst into flame; if the Li-ion cell over-discharges, the pressure in the cell will increase, the electrolyte will decompose and thereby the core capacity and durability will be degraded; as a result, the serving time of the Li-ion battery will become shorter and shorter. For other cells, such as ferric-ion cells, over-charge or over-discharge will also deteriorate the characteristics of the cell and shorten the service life of the cell. A cell protection circuit is designed to prevent over-charge or over-discharge and thereby protect the cell against deterioration. As electronic devices develop towards the trend of miniaturization, integrated circuits specially designed to protect cells emerge as the times require. At present, one integrated circuit for cell protection can only protect up to 4 cells connected serially. In applications where more than 4 cells are connected serially, a plurality of integrated circuits for cell protection can be connected through a complex peripheral circuit to implement protection of cell. However, the drawbacks of such an approach include: high cost; complex peripheral circuit; poor expandability and testability in actual applications; excessive exterior components, large footprint, and adverse effect to system integration; uneven power consumption among the cells connected serially, etc.
FIG. 1 shows a typical circuit connection for protection of a battery composed of seven cells connected serially, which is implemented by connecting seven individual integrated circuits for cell protection through a peripheral circuit.
The working principle of an integrated circuit for protection of cells is as follows: the terminal Co of the integrated circuit for protection of cells is a grid connection terminal of a NMOSFET for controlling charge, while the terminal Do is a grid connection terminal of a NMOSFET for controlling discharge. In normal state, both the terminal Do and the terminal Co are at high level, the exterior NMOSFETs for controlling charge and discharge are in ON state, and cells are in chargeable and dischargeable state. When the charging voltage on the cells reaches to the upper limit for the cells, the output level at the terminal Co will change from high level to low level after some time delay, and thereby switch off the exterior NMOSFET for charging, i.e., switch off the charging circuit; when the discharging voltage of the cells reduces to the lower limit for the cell, the output level at the terminal Do will change from high level to low level after some time delay, and thereby switch off the exterior NMOSFET for discharging, i.e., switch off the discharge circuit.
Through the peripheral circuit connection, over-charge and over-discharge of the cells will be treated separately, to implement cell protection against over-charge and over-discharge, as follows:
In normal state, both the terminal Co and the terminal Do of each integrated circuit for cell protection are at high level, the NMOSFETs V5, V6, and V7 for controlling discharge and the NMOSFET V8 for controlling charge are in ON state, and the entire battery is in chargeable and dischargeable state.
When discharging, if any integrated circuit for cell protection detects over-discharge of a cell, the terminal Do of the integrated circuit will change to low level after some time delay, and thereby switch the exterior PMOSFET V2(V2A, V2B . . . or V2G) to ON state, transmit a high level potential related to ground to NMOSFET V4 to switch on V4, and transmit a ground potential to the NMOSFETs V5, V6, and V7 in the exterior circuits connected in parallel for discharge control and thereby switch them from ON state to OFF state to switch off the discharge circuit and achieve protection against over-discharge.
When charging, if any integrated circuit for cell protection detects over-charge of a cell, the terminal Co of the integrated circuit will change to low level after some time delay, and thereby switch the exterior PMOSFET V1(V1A, V1B . . . or V1G) to ON state, transmit a high level potential related to the negative terminal of the charger to NMOSFET V3 to switch on V3, and transmit a potential at negative terminal of the charger to the NMOSFET V8 in the exterior circuit for discharge control to switch it from ON state to OFF state and thereby switch off the charge circuit and achieve protection against over-charge.
In functionality, the circuit described above has attained the object of protection for seven cells connected serially. However, it has the following drawbacks: the peripheral circuit is complex; a large number of components are used; a two-stage treatment approach is employed for level shifting, and therefore increases circuit cost and difficulties in testing; the separation between charge circuit and discharge circuit increases complexity in application; after over-charge protection or over-discharge protection is activated, the power consumption of the circuit is higher than the power consumption of the in normal state, due to the existence of the peripheral circuit; especially, the MOSFETs in the peripheral circuit must be components endurable to high voltage; therefore, in applications where more cells are connected, the circuit will be more complex, in consideration of voltage endurance of the components.